This invention relates to a position-sensing potentiometer arrangement for sensing the position of a piston within a linear actuator, more particularly, a linear actuator such as a work cylinder which can be used, for example, to control the door-movement of a public transit vehicle where it is required to precisely determine the door position at any particular time. Such a work cylinder can also be used in other applications requiring a precise determination of the position of an object to be controlled by the work cylinder.
In order to determine each respective position of the piston to a precise degree, linear actuators have typically provided an inductance measuring arrangement wherein a coil is wound around a cylinder; such an arrangement proving to be costly in both the manufacturing and maintenance phases of operation. It is conceivable, however, to provide a linear actuator using a potentiometer-type measuring arrangement wherein a resistance element is secured to a stationary portion of the linear actuator and a sliding contact is secured to the movable piston rod. One example of a potentiometer-type measuring device can be found in U.S. patent application Ser. No. 647,092, which has matured into U.S. Pat. No. 4,566,196 assigned to the assignee of the present invention. The invention contained therein, however, is directed toward an externally attachable position measuring arrangement, and there are certain industrial and mechanical applications where space requirements are such that the position-sensing arrangement occupies very little space in addition to that of the work cylinder. In work cylinder applications adhering to strict external space requirements, it is still necessary to provide a precise position indication, even where the piston stroke can be very large. With the stroke of the piston, sliding contacts mounted on a rod-shaped support must be moved along the path of the resistance element between the two end points of the resistance element. With each change of the distance of the sliding contacts to the two resistance points, the voltage picked up by the potentiometer will change, thus indicating the position.
If the work cylinder has a great overall length, the position-sensing potentiometer arrangement must have a great overall length as well. This means that the resistance element and the support, in the form of a rod for the sliding contact, must have a length that corresponds to the maximum length of the piston. Due to the greath length of the support for the sliding contact, it could happen that the support bends and partially rests upon the resistance surface. The function of the potentiometer would be greatly affected by this circumstance. It is possible, of course, to achieve greater rigidity of the support by dimensioning the cross-section of the support accordingly. However, this would be associated with an undesirable, and for many applications, no longer acceptable, enlargement of the entire potentiometer. Additionally, such dimensioning would increase the weight of the device, thereby affecting the force and time-efficiency of the overall work cylinder.